The domain of the invention is Global Navigation Satellite Systems (GNSS), such as Global Positioning System (GPS), Galileo or GLONASS. The invention relates to the acquisition of signals output from satellites in such a positioning system by a receiver with at least one channel comprising several code correlators.
The invention is applicable to wide band spread spectrum signals, for example of the Code Division Multiple Access (CDMA) type known to those skilled in the art, typically but not limitatively for GPS/GNSS signals, either with Binary Phase Shift Keying (BPSK) modulation referring to binary modulation by phase shift or Binary Offset Carrier (BOC) and derivatives of these.
The acquisition of signals in a GPS, Galileo or GLONASS type GNSS system consists of aligning the signal output from a satellite with a replica signal generated locally by the receiver. To succeed this alignment, the receiver must:                Apply a compensation for the Doppler shift of the carrier for the satellite signal being searched for relative to the nominal frequency of said satellite signal carrier, this shift depending on the apparent speed of the satellite relative to the receiver and the difference between the theoretical reference frequency and the real frequency supplied by the local oscillator of the receiver; this shift can be estimated with more or less uncertainty depending on available information (satellite almanac, receiver speed, aging of the oscillator).        Shift the local code until it is aligned onto the code emitted by the satellite. This alignment is confirmed when the result of the correlation integrated over a more or less long period exceeds a detection threshold, the definition of the integration time and detection thresholds depending on the power of the received satellite signal and the presence and power of interference.        
Therefore the search for satellite signals is made in the two dimensions “time-frequency”. It is done by one or several correlators, each correlator testing a time-frequency assumption by correlating the received signal compensated for the Doppler shift with the locally generated replica signal.
Finally, the GNSS signal acquisition or re-acquisition times depend on:                The size of the satellite signal “time-frequency” search domain;        The number of correlators used for the search;        The time necessary to determine if the signal is present or absent for a “time-frequency” assumption.        
Techniques have been developed to reduce signal acquisition times for a signal output by a satellite in a satellite positioning system.
The following description is illustrated with reference to FIG. 1 that is a principle diagram of a channel of a receiver in a satellite positioning system using multi-code correlation and multi-frequency correlation.
A GNSS receiver has several channels that it uses to simultaneously acquire and then track several satellites. A first way of reducing acquisition times is to increase the number of code correlators 12 per channel, by introducing a shift register 10 to generate several replica signals S1, Sc, SN, for example N replica signals, shifted by a code moments (conventionally α=½), which simultaneously tests one code assumption for each given frequency, namely N assumptions; this is referred to as multi-code correlation.
The number of correlators can also be extended by frequency processing of the results output from code correlators 12; this is referred to as post-detection multi-frequency correlation, since the frequency processing is done on the output from the multi-code correlation results, unlike architectures described for example in the article “A Fast Satellite Acquisition Method” by David Akopian presented in the ION GPS 2001, Sep. 11-14 2001, Salt Lake City.
It can be broken down as follows:                sampling of correlation results at period T/M        the use of frequency-augmented correlators 14 for the convolution of outputs from code correlators with carriers shifted relative to the central frequency by ωm=m/(aT) where mε[−M; +M]        summation of M samples to reconstitute complex outputs (I,Q) on the coherent integration period T.        
For each code correlator 12, the result obtained is thus complex outputs (I,Q) of 2M+1 frequency-augmented correlators 14 shifted in steps of m/(aT) around a central frequency that is the nominal frequency of the carrier corrected by the Doppler shift.
With this post-detection multi-code and multi-frequency correlation mechanism, the search domain in time-frequency is scanned by simultaneously testing outputs from N×(2M+1) frequency-augmented correlators 14, which reduces satellite signal acquisition or re-acquisition times.
However, the use of a post-detection multi-frequency correlation has the disadvantage that the replica signal code generation speed is proportional by a factor f to the nominal carrier frequency compensated for the Doppler shift, but not to carriers shifted by ωm relative to this reference.
For example, satellites in the GPS system emit on at least two carriers L1=1575.42 MHz and L2=1227.6 MHz. The phase of carrier L1 is modulated by a first spread code called C/A (“Coarse Acquisition”) and the navigation message, and it is modulated in quadrature by a second encrypted spread code called P(Y) and the navigation message. The phase of carrier L2 is modulated by at least the spread code P(Y) and the navigation message. The throughput of the C/A code is 1.023×106 bits or moments per second, while the throughput of the P(Y) code is 10.23×106 bits or moments per second.
Therefore the scale factor f is typically 1/1540 for the C/A code on carrier L1, 1/154 for the P(Y) code on carrier L1 and 1/120 for the P(Y) code on carrier L2. The result is that the consequences of an unsuitable compensation for the Doppler shift will be greater for acquisition of the P(Y) code than for acquisition of the C/A code.
The generation speed of the replica signal Sr is coherent with the nominal frequency of the carrier compensated for the Doppler shift. On the other hand, the generation speed of the replica signal Sr is not coherent with the carriers shifted by ωm relative to this central frequency.
This incoherence is not a problem provided that the integration of the increased correlator outputs is only for a short duration. On the other hand, in the presence of interference or jammers, the signal to noise density ratio C/N0 drops and integration times Ti have to be longer to determine if the signal is present or absent for a given code-frequency assumption and this incoherence becomes a problem.
Thus, considering acquisition of GPS P(Y) signals on carrier L1 for a frequency-shift of ωm Hz, the error on the code generation speed is ωm/154 code moments per second. This error then generates a code slip with time, in other words a progressive shift of the code generated relative to the signal code. This is a mistake induced by the shift relative to the central frequency of the frequency-shifted carriers used in frequency-augmented correlators 14.
Considering a space between code correlators 12 equal to a half-code moment, if the integration time for a first frequency-augmented correlator 14 associated with a first code correlator 12 shifted by ωm, exceeds Ti>77/ωm, the signal will be displaced to a second frequency-augmented correlator 14 using the same frequency-shift as the first frequency-augmented correlator 14 and being associated with a second code correlator 12 that is shifted by the code spacing of the first code correlator 12 with which said first frequency-augmented correlator 14 is associated.
If nothing is done to correct the effects of this error induced by the shift relative to the central frequency of the frequency-shifted carriers used in the frequency-augmented correlators 14, the N frequency-augmented correlators 14 for which the frequency-shift is such that |ωm|>77/Ti become unusable and the signal acquisition will take longer because of the smaller number of useful frequency-augmented correlators 14.
FIG. 2 shows an example of signal detection probabilities (in %) as a function of the index m of the frequency-shift between the central frequency with which the code generation speed is coherent and the real frequency of the searched signal. This figure shows the consequences of lack of compensation of the induced error for acquisition of P(Y) signals on L1 with a low signal-to-noise ratio. In this case, the necessary integration duration is of the order of several seconds, and the induced error for significant frequency-shifts exceeds the P(Y) code half moment and the integration results are invalid: almost no detection is possible for these frequency-augmented correlators 14.
Consequently, the satellite signal acquisition time may be significantly longer due to the small number of useful correlators.